In recent years, improvements in measurement precision of an optical characteristic inspection apparatus has been needed in precise measurement in the manufacture of electronic parts such as a semiconductor product, or in the field of nanotechnologies.
Interactions between material and light falling on and going through the material can be classified into five types: specular reflection, diffuse reflection, (specular) transmission, diffuse transmission, and light absorption. More specifically, reflection and transmission phenomena include specular reflection in which an incident angle and a reflection angle are equal to each other, (specular) transmission in which an incident angle and a transmitted light angle are equal to each other, and scattering in which, for one incident angle, only reflected light for the opaque material, or both reflected light and transmitted light for the transparent material are produced toward a large space (scattering phenomenon which is a combination of diffuse reflection and diffuse transmission).
Conventionally, in order to measure specular reflectance and (specular) transmittance, different accessories are used to measure relative reflectance and absolute transmittance. A drawback of this method lies in the fact that their measurement precisions of measured quantities differs from each other. In order to overcome the drawback, the present inventor has developed an apparatus in which transmittance measurement and reflectance measurement are integrated (see Japanese Patent Application Laid-Open Publication No. 2004-257956; Japanese Patent Application Laid-Open Publication No. 2004-45065; and Japanese Patent Application Laid-Open Publication No. 2006-234681).
Light scattering phenomena due to material include complete scattering of light scattered uniformly in a whole space (4π space) and partial scattering of light scattered in a certain partial space. A loosely-packed fine powder is one example of the former complete scattering, while there are numerous examples of the latter around us. More specifically, a ceramic tile, a painted surface, fabric (warp and weft), and a paper surface (paper fiber has a net-like structure) are examples of the latter. The specular reflection and (specular) transmission phenomena can be considered as a limit of “a certain partial space” in this partial scattering. As will be seen from the above examples, in order to measure scattering, it is necessary to measure both total spherical scatter (TSS) and scattering anisotropy (BSDF: bidirectional scatter distribution function).
In the field of optical measurement of scattered light from a sample, a scatterometer using a hemispheroidal mirror is known. A scatterometer using an integrating sphere and a scatterometer using a gonioreflectometer are also known. A scatterometer using an imaging hemisphere and a scatterometer using two ellipsoidal mirrors (also called “seagull-like scatterometer”) (see U.S. Pat. No. 5,210,418) are also known.
The present inventor suggested a double ellipsoidal optical system structure and developed an optical characteristic measuring apparatus. The present inventor already developed an apparatus using an optical system using a double elliptical cylindrical mirror (see Japanese Patent Application Laid-Open Publication No. 2004-257956). In addition, the present inventor developed an apparatus for measuring absolute reflectance and absolute transmittance using an optical system constituted by combining two ellipsoidal mirrors, not by combining elliptical cylinders (see Japanese Patent Application Laid-Open Publication No. 2004-45065 and Japanese Patent Application Laid-Open Publication No. 2006-234681). Those apparatuses are each provided with a double ellipsoidal mirror composed of two spheroidal mirrors, and two beam switching mirrors and a sample are disposed at each focal point.
Furthermore, the present inventor developed an apparatus capable of rotating by a predetermine angle a light-receiving ellipsoidal mirror constituted by a double ellipsoidal optical system, and rotating by a small angle a beam switching mirror located at a focal point of the ellipsoidal mirror, and measured anisotropy of scattered light (see Kawate E., “Measurement method of optical scatter using a STAR GEM as a scatterometer” In proceedings of the SPIE, vol. 7065, 2008, 706515-1-706515-9 and FIG. 14 in Japanese Patent Application Laid-Open Publication No. 2010-276363). The double ellipsoidal optical system has a structure in which one focal point of the light-entering ellipsoidal mirror E1 and one focal point of the light-receiving ellipsoidal mirror E2 are used as a common focal point F0, and the remaining focal points of the ellipsoidal mirrors E1 and E2 are defined as F1 and F2, respectively, these three focal points F0, F1, and F2 are aligned with each other.
In addition, the present inventor developed an apparatus which rotates by a predetermined angle a light-receiving ellipsoidal mirror constituted by a double ellipsoidal optical system, and detects scattered light collected on a focal point of the rotated ellipsoidal mirror, thereby making it possible to measure total spherical scatter of a sample (see Japanese Patent Application Laid-Open Publication No. 2010-276363). In Japanese Patent Application Laid-Open Publication No. 2010-276363, first and second ellipsoidal mirrors constituting a double ellipsoidal optical system each has a structure made from a plate-like member or belt-shape member, which has a predetermined thickness (see FIG. 2 Japanese Patent Application Laid-Open Publication No. 2010-276363).